A conventionally typical fuel tank is illustrated in FIG. 9 hereof. The fuel tank 100 is in the form of a container comprised of a lower case 101, a lower inner case 102, and an upper case 103 laid one over the other and connected together by a round-gathering process as at 104.
Reference is made next to FIG. 10 hereof illustrating the round-gathered part or gathering 104 in enlarged cross section. The lower case 101 has an outer peripheral end 105 in the form of a flange simply bent radially outwardly. The lower inner case 102 has an outer peripheral end 106 around which an outer peripheral end 107 of the upper case 103 is twined. Both outer peripheral ends 106, 107 are then twined around the outer peripheral end 105 of the lower case 101. As a result, the outer peripheral end 107 of the upper case 103 becomes longer than the other peripheral ends 105, 106.
Referring next to FIGS. 11A and 11B hereof, discussion will be made as to a mode of forming the gathering 104 through the round-gathering process. Note that the process is performed with the upper case 103 and the lower case 101 turned over.
As shown in FIG. 11A, the outer peripheral end 107 of the upper case 103 is placed on a die 111, followed by placing the outer peripheral end 106 of the lower inner case 102 on the outer peripheral end 107 of the upper case 103. Then, on that outer peripheral end 106 is placed the outer peripheral end 105 of the lower case 101. A punch 112 is then lowered as shown by arrow A. As a result, the outer peripheral end 107 of the upper case 103 is bent as shown by arrow B.
Thereafter, the half-finished product is placed on a separate die 114, as shown in FIG. 11B. This is followed by lowering a separate punch 115 as shown by arrow C. As a result, the outer peripheral ends 105-107 are round-gathered unitarily as shown by arrow D. The half-finished product is then turned over again to its original posture to thereby provide the gathering 104 as shown in FIG. 10.
Continuously, the product is transferred to a coating line, where it is applied with a coat to become a finished product. As can be appreciated, for the product to be finished, it must go through several different processes, namely, round-gathering, transfer to the coating line, and coating. The product transfer to the coating line is particularly tedious and time-consuming, increasing the cost of manufacture of the product.
One known way to reduce the cost of manufacture is to use a pre-coated steel sheet as a starting material and to plastically work such a steel sheet into a container. However, the known way has a drawback in that when the pre-coated steel sheet is largely wrinkled during the plastic working, the coat film thereon cracks up and partially comes off.
As a measure to overcome the problem, JP-2004-298906A proposes a drawing method for plastically working a pre-coated steel sheet in such a manner that the coat thereon does not come off.
The proposed drawing method comprises the steps of drawing a pre-coated steel sheet under various conditions to identify a coat film peel-off state, performing a drawing simulation on an original, non-coated steel sheet under the same conditions, estimating from the results of the preceding two steps a minimum degree of warp that produces partial peeling off of a coat film, and adjusting, during subsequent actual working of a pre-coated steel sheet, a corner cut amount and drawing conditions such that warping at a position where coat film peel-off is most likely to arise is kept smaller than the minimum degree of warp.
For the drawing simulation, a computer simulation called a Computer Aided Engineering (CAE) analysis is employed for estimating warps to be generated on various parts of the non-coated steel sheet.
In the proposed drawing method, however, since the CAE analysis on various parts of the original steel sheet is essential, an extra cost for CAE analysis is required.
Further, because the drawing conditions must be adjusted such that warps are kept smaller than the minimum degree of warp that causes partial peeling off of a coat film. This may not only limit a per-cycle amount of drawing but also may require frequent adjustment operations, thus requiring increased man hours. To sum up, in the proposed drawing method, an increased cost of manufacture is highly likely due to the extra analysis cost combined with the increased man hours.
Consequently, there has been a demand for an improved technique for forming a container half from a coated steel sheet.